In a water-cooled nuclear reactor, when a cooling water is supplied no longer into a nuclear reactor pressure vessel because of stop of supply of the cooling water and/or fracture of piping, the reactor water level comes down to expose a reactor core and possibly results in insufficient cooling of the reactor core. Preparing for such a case, a signal indicating the drawdown of water level is used to automatically subject the nuclear reactor to emergency stop, and a cooling material is poured by an emergency core cooling system (ECCS) to cover the reactor core with water and cool it, thereby preventing the core meltdown accident.
However, it takes a certain amount of time to input the cooling material, and it can also be envisioned that, though at a very low probability, the emergency core cooling system does not operate and another device for pouring water to the reactor core is not available. In this case, the water level in the nuclear reactor pressure vessel remains at the lowered level, and sufficient cooling of the exposed reactor core is not performed any longer to increase the temperature of fuel rods by the decay heat continuously generated even after the stop of the nuclear reactor, finally leading to a core meltdown.
When falling into such a situation, the molten corium (corium) at a high temperature melts down to a lower portion of the nuclear reactor pressure vessel, and melts and penetrates the lower portion of the nuclear reactor pressure vessel and finally falls onto the floor inside the containment vessel. The molten corium heats the concrete laid on the containment vessel floor and reacts with the concrete when the contact surface between them becomes a high temperature state to generate a large quantity of a non-condensable gas such as carbon dioxide or hydrogen and melt and corrode the concrete.
The pressure of the generated non-condensable gas can be reduced to some degree by cooling it in a suppression pool, but if the quantity of the generated gas is large, its pressure cannot be sufficiently reduced even in the suppression pool. This can result in an increase in pressure inside the containment vessel to damage the nuclear reactor containment vessel, and the melting and corrosion of the concrete can damage the containment vessel boundary. In short, a reaction occurring between the molten corium and the concrete and continuing for a predetermined period leads to damage of the containment vessel and can release a radioactive material inside the containment vessel to an external environment.
From such a viewpoint, in order to suppress the reaction between the molten corium and the concrete, it is necessary to cool the molten corium to decrease the temperature of the surface of a bottom portion of the molten corium in contact with the concrete to a corrosion temperature or lower (1500 K or lower for a typical concrete), or to prevent the molten corium from coming into direct contact with the concrete. As a representative of the latter means, there is a so-called molten corium holding device (core catcher). The molten corium holding device is a facility which receives the falling molten corium by a heat-resistant material and cools the molten corium in combination with a water pouring systems.
However, a period of about 10 minutes may be required until the cooling water is supplied from the water pouring systems, and it is necessary to hold the molten corium only by the molten corium holding device during this period. Accordingly, the molten corium holding device is required to have a very high heat resistance.
It has conventionally been tried that the molten corium holding device is composed using concrete containing as main components calcium oxide and silicon oxide or the molten corium holding device is composed using tiles made of a high melting point material. However, the temperature of the molten corium holding device rapidly increases from room temperature to 2000° C. when holding the molten corium. Therefore, an optimal molten corium holding device is not provided yet at present because various factors of damage work in combination, such as a problem of damage due to the thermal stress generated at the temperature increase, a problem of the reaction of the molten corium with the heat-resistant material constituting the molten corium holding device, a problem of a so-called jet impingement that the molten corium spouting in a jet form locally collides against the surface of the heat-resistant material to cause melting and corrosion and so on.